Ferrite core inductor variable by altering direction of steady magnetic field



Sept. 29, 1964 A. w. SIMPSON 3,151,305

FERRITE CORE INDUCTOR VARIABLE BY ALTERING DIRECTION OF STEADY MAGNETIC FIELD Filed Jan. 24, 1961 2 Sheets-Sheet 1 AM. SMPSON \NVENTOR BY Uw/iam if. ATTORNEY P ,1964 A w. SIMPSON 3,151,305

FERRITE CORE IN DUCTOR VARIABLE BY ALTERING l DIRECTION OF STEADY MAGNETIC FIELD Flled Jan. 24. 1961 2 Sheets-Sheet 2 BANDI BANDIII N=4 msc HORIZONTAL DISC 'HORIZONT w w v v 2 2 U U a. 5

82 a :3 O 7 O m u! E 1 E 60 H I- z 2 50 i g QVA N c a; CURVE a: C E 0 30 v 90 0 3O 6O 90 e DEGRE S e DEGREES BANDI BANDHI N 4 DISC =30 N-2 DISC =30 Q VALU E- VALUE 40 ON CU R E CU R E O 30 6O O 30 6O 90 e DEGREES e DEGREES RESONANT FREQU EN CY Mc/s RESONANT FREQUENCY Mc/s United States Patent 3,151,395 FERRITE CORE INDUCTGR VARIABLE BY ALTERING DIRECTION OF STEADY MAG- NETIC FIELD Anthony William Simpson, Marlow, The Green, Wootton, England, assignor to The Plessey Company Limited, London, England, a British company Filed Jan. 24, 1961, Ser. No. 84,745 Claims priority, application Great Britain, Jan. 23, 1%tl,

6 Claims. (Cl. 336-116) This invention relates to a variable inductor and par ticularly to tunable circuits and devices employing such a variable inductor.

The use of so-called ferrite materials in coils which are intended to operate at high radio frequencies for example above 1 megacycle per second has been limited by core losses. Attempts have been made to reduce these lossesby utilising materials which have grain orientation. However, in the case of ferrite materials the orienting of the grain has been found to be difiicult and expensive to achieve.

It is. an object of the present invention to provide a variable inductor which is relatively simple to construct and which avoids the need to apply variable bias fields to the ferrite materials in order to vary the inductance of thein'ductor.

' According to the present invention a variable inductor comprises a coil, 21 core of a ferro or ferrimagnetic material within the coil, external means for producing a steady magnetic fieldin the core and means for varying the angle between the axis of the coil and the steady magnetic field through the core.

In accordance with a second aspect of the present invention there is provided a variable inductor including a coil carried on a core of ferro or ferrimagnetic material andrneans for subjecting the core to a steady magnetic field whose direction relative to the axis of the coil is selectively variable.

For a better understanding of the invention reference will be made to the accompanying drawings in which:

' FIGURE 1' is a diagrammatic perspective view of an inductor incorporating the features of the present invention, FIGURE 1A is an elevation and FIGURE 18 a plan view to show the relationship of the angles a and 6 to the inductor of FIGURE 1,

FIGURES 2 and 3 are graphical plots showing the relationship between resonant frequency of a tunable circuit including the coil of the inductor of FIGURE 1 and the angle between the static magnetic field and the axis of the coil of theinductor of FIGURE 1, when rotation takes place in the plane of the field,

FIGURES 4 and 5 are graphical plots showing the relationship between the direction of the steady magnetic field and the axis of the inductor when the core is inclined at a'fixed angle to the plane of rotation of the ma ne i ld FIGURE 6 is a schematic representation of a tuning device incorporating the inductor of FIGURE 1.

' Referring now to FIGURE 1 the variable inductor includes a pair of spaced apart magnets 1 of a ferrite material and a core 2 in the form of a flat plate, i.e. disc of a second ferrite material located in the gap between the facing ends of the magnets 1. A coil 3 is wound on the disc. The magnets 1 are mounted upon a suitable rotatable support means 4 whereby the magnets 1 can be rotated about an axis 5. The axis 5 intersects the centre of the core 2 which is not rotatable. The arrow headed lines 6 indicate the direction of movement of the magnets 1.

' The number of turns of the coil 3 on the core will be determined in accordance with the actual range of ire- 3,151,305 Patented Sept. 29, 1964 quencies which it is desired that the variable inductor should be capable of tuning. The material from which the magnets l are made and the separation between the opposing faces of the magnets are selected so that the steady field producedbetween the two magnets 1 is suiticient completely to saturate the core 2. A suitable material for the magnets is a barium ferrite.

The magnetisation of the core in the absence of a magnetic field produced by the coil in consequence of a current passing therethrough will be wholly in the direction of the magnetic field produced by the magnets 1. The magnetic field produced in the core in consequence of energisation of the coil, although small compared with that produced by the steady field will affect the overall magnetisation of the core. Thus when these two magnetic fields are perpendicular the effect of the coil field will be to rotate the direction of magnetisation of the core in such manner that a small component of the core magnetisation will be produced in the direction of the core field thereby increasing the magnetisation of the core in this direction. When the two fields are parallel the field due to the coil cannot have any effect as the steady field is sutficient to saturate the core. The change in magnetisation of the core is equivalent to a change of Yfiux in the coil and thus a change in the permeability.

Under these conditions it may be shown that the eifective permeability n of the material forming the core is given by the relationship 1+A sin 6 where 0 is the angle betweenthe direction of the steady magnetic field through the coil and the axis of the coil on the core and A is a constant relative to a particular inductor which includes factors such as the effective applied steady field and the magnetisation of the core material.

It will be seen from the above equation that the permeability ,u. is unity when the angle 6 is zero and that it attains a maximum value when 0 is Since the magnetisation varies from its maximum to minimum value within 90 of rotation of the angle between the externally produced field and that produced by the coil, a rotation of will involve two variations between maximum and minimum, while a rotation of 360 will produce four variations between the maximum and minimum. The magnetic field produced by the magnets I, should be large enough to produce a degree of saturation in the core which is known as Technical Saturation or a saturation which is sufficient to produce a Q in the tunable circuit which is acceptable in a particular application of the inductor. This maximum value will be related to the magnetisation of the core and to the magnitude of the effective applied field.

FIGURES 2 and 3 are graphs which show the relationship between the magnitude of the resonant frequency in mc./s. of a tunable circuit including the coil of the inductor and the angle between the steady magnetic field and the field produced by the coil 3 in the core 2. In order to produce the graphs of each of FIGURES 2 and 3 the core 2 was horizontal and was located in the plane of rotation of the steady field. The curve shown in FIGURE 2 represents the relationship when-the coil 3 consisted of four turns. it was found that a variation of the angle 0 from 0 to 90 produced a substantially linear variation of the resonant frequency of the tunable circuit within the range 43 rnc./s. to 74 mc./s. i.e. a frequency swing of 31 mc./s. The numbers on the curve represent the Q value of the resonant circuit.

FIGURE 3 represents a similar curve to that of FIG- URE 2. However the number of turns of the coil on the core were reduced to two. It was found that the inductor was capable of tuning the circuit over a frequency range which was considerably higher than that of FIGURE 2. It'will be observed that the relationsoip between resonant frequency of the circuit to be tuned and the angle be- 7 3 13 tween the steady field and the coil field is likewise substantially linear. A variation of the angle 0 from 0 to 90 produced a variation of the resonant frequency within the range 150 rue/s. to 220 mc./s. The frequency range tunable by the inductor having four turns on its coil is that identified as Band I for the purpose of television reception, whereas the tunable range of the inductor having two turns on its coil is equivalent to that referred to as Band Iii.

An inductor was constructed having a coil consisting of a single turn of Wire Wound round a diameter of a core in the form of a disc. It was connected across a 30 ,u uf. capacitor to form a tunable circuit which resonated at 200 mc./s. with a Q of 70 when a steady magnetic field was applied perpendicular to the plane of the coil, and when the static magnetic field was applied in a direction parallel to the plane of the coil the resonant frequency was 240 mc./s. and the Q 75. With the steady field at intermediate angles to the coil intermediate values of the resonant frequency were obtained. It was therefore possible to tune the circuit to any desired frequency in the range 200 to 240 mc./s. by rotating the steady magnetic field. A similar disc having 4 turns of wire was found to provide an effective permeability of nearly 3 for the steady field applied perpendicular to the coil axis. Furthermore it was found that such a system could tune a circuit from 45 mc./s. to 73 mc./s. with a Q always greater than 90.

It can be shown that if the plane of the disc is tilted at an angle a with respect to the plane of rotation of the steady magnetic field relative to the direction of the field produced by the coil on the core that the effective permeability [A is given by the following relationship:

sin 6 cos a ([sin 6' cos 04-1-003 0 1) FEGURES 4 and 5 illustrate respectively the relationship between the tuned resonant frequency and the angle 6 between the steady magnetic field and the coil magnetic field when the number of turns on the core are 4 and 2 respectively,and the angle a subtended by the core with respect to the plane of rotation of the steady magnetic field is fixed at 30. it will be observed that the relation between the frequency and the angie 0 is linear and that a greater degree of linearity of the tuning curves is obtained as compared with the tuning curves of FIGURES 2 and 3. Consequently, in view of the linear relationship of the tuning curves of the inductor, the inductor provides a convenient device for tuning a television receiver to one end 8 of the element 7, while the other end thereof is connected to terminal 9. The other end of the element 7 is connected to a second terminal 10. A tapping 11 on the coil 3 is connected with the wiper arm 6, whereby when the wiper arm is in engagement with the element 7 three of the turns of the coil are short circuited, thereby leaving single turn in circuit. When the wiper arm 6 is moved out of engagement with the element all of the turns of the coil are in circuit. A capacitor 12 is connected across the ends of the coil B, the capacitor 12 forming with the coil a tunable circuit. The position of the switch wiper arm 6 can provide a direct reading of the tuning frequency if suitably calibrated scales are provided.

The actual shape of the core is not critical. The shape is in fact only important in that it governs the demagnetisation factor. The greater the ratio of the thickness of the core to the diameter the greater is the demagnetisation factor, and therefore the smaller the maximum effective permeability, consequently this ratio should be made as small as possible. It has been found that in the case of a core in the form of a disc the ratio between the diameter and the thickness should be preferably within the range 15:1.

A lower demagnetisation factor and more uniform magnetisation could be obtained by using a flat oblate spheroid instead of a disc.

One of the causes of losses in the ferrite is the hysteresis loop of the material. If the ferrite is completely saturated the loop is closed and these losses disappear. Therefore the steady field must be at least sufiicient to saturate the material, and materials which saturate easily are desirable.

The other main cause of losses is the gyromagnetic resonance phenomenon. For a disc shaped specimen this resonance occurs at a frequency 1 given by f='y /BH where B and H are the magnetisation and applied fields respectively and are measured in gauss and oersteds, and 'y is a constant equal to 2.8 if the frequency is measured in megacycles. In order to avoid these losses the operating frequency must be well below the resonance frequency. For a typical ferrite B is of the order 2500 gauss and if the applied field H is 200 oersteds the gyrornagnetic frequency becomes 2000 mc./s. and this source of loss can be ignored for all normal applications.

It has been found that a suitable magnitude for the steady field is preferably in the range 450-750 oersteds, a convenient value being 550 oersteds.

Materials for the cores are given in the following table:

I The tuning device includes a switch which has a wiper arm 6 which co-operates with an arcuate element 7. The

arm 6 is mechanically linked to the support of the magnets of the inductor by a means not shown. The coil 3 consists of four turns. One end of the coil is connected.

Using a core in the form of a disc 1.4 cm. diameter and of a thickness of 0.1 cm. with a four turn coil and an external field of 500 oersteds, the lower frequency was 45 1nc./s.

In the above mentioned compositions the total of the Zn() and Nit) should add to 50 mol percent. Preferred ranges are ZnO 532 and NiO 45-18 respectively. Conveniently the Fe O content should be less than 50 mol percent whilst the sum of the MgO-l-MnO should lie within the range 0 to 10 mol percent. The discs can be prepared by known ceramic techniques.

Although the foregoing description has indicated that the magnets l are rotated relative to the core and coil in order to move the steady, or external, magnetic field, it

will be appreciated that the equivalent magnetic effect could be obtained by moving the core, and coil, or moving the coil relative to the core.

What I claim is:

1. A variable inductor comprising a core of a ferrite material, a coil arranged upon the core, external means for producing a steady saturating magnetic field within the core, and means for enabling the angle between the direction of the saturating field and the axis of the coil to be varied.

2. A variable inductor comprising a disc like core of a ferrite material, a coil arranged upon the core, external means for producing a steady saturating magnetic field within the core, and means for enabling the variation of the angle between the direction of the saturating field and the axis of the coil.

3. A variable inductor comprising a disc like core of a ferrite material, a coil arranged upon the core, external means for producing a steady saturating magnetic field within the core in a direction parallel to the plane of the core, and means for enabling the angle between the direction of the saturating magnetic field and the axis of the coil to be selectively varied.

4. A tunable inductive circuit including a core of ferrite material, a coil arranged upon the core, a capacitor connected in parallel with the coil, external means for producing a steady saturating magnetic field within the core, and means for varying the angle between the axis of the coil and the direction of the saturating field.

5. A tunable inductive circuit including a core of ferrite material, a coil arranged upon the core, a capacitor connected in parallel with the coil, a pair of magnets of ferrite material located on opposite sides of the core and arranged to produce a steady saturating magnetic field through the core; rotatable support means for the magnets whereby the pair of magnets can be rotated relative to the core to vary the angle between the steady saturating eld and the axis of the coil.

6. A variable inductor including a pair of magnets of ferrite material, support means for mounting the magnets in spaced apart relationship, so that they produce a steady magnetic field along a first direction, means for rotatably mounting the support means so that the latter is rotatable about a first axis transverse to said direction, a core of ferrite material located between the magnets of said pair and symmetrically arranged with respect to said first axis, so that the magnet pair produce a steady saturating field within the core, a coil so arranged on the core that its coil axis intersects the first axis whereby rotation of the magnet pair varies the angle between the axis of the coil and the direction of the steady magnetic field.

References Cited in the file of this patent UNITED STATES PATENTS 2,159,754 Wohlfarth May 23, 1939 2,200,263 De Kramolin May 14, 1940 2,462,423 Polydorotf Feb. 22, 1949 2,581,202 Post Ian. 1, 1952 2,804,617 Polydorotf' Aug. 27, 1957 2,860,313 Israel Nov. 11, 1958 2,882,392 Sands Apr. l4, 1959 2,915,637 McAdam Dec. 1, 1959 

1. A VARIABLE INDUCTOR COMPRISING A CORE OF A FERRITE MATERIAL, A COIL ARRANGED UPON THE CORE, EXTERNAL MEANS FOR PRODUCING A STEADY SATURATING MAGNETIC FIELD WITHIN THE CORE, AND MEANS FOR ENABLING THE ANGLE BETWEEN THE DIRECTION OF THE SATURATING FIELD AND THE AXIS OF THE COIL TO BE VARIED. 